high-solid anaerobic digestion of sewage sludge under mesophilic conditions: feasibility study

7
High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study Nina Duan, Bin Dong , Bing Wu, Xiaohu Dai National Engineering Research Center for Urban Pollution Control, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China article info Article history: Received 10 September 2011 Received in revised form 22 October 2011 Accepted 25 October 2011 Available online 3 November 2011 Keywords: Anaerobic digestion Sewage sludge High-solid Mesophilic abstract Feasibility of high-solid anaerobic digestion of sewage sludge was investigated in single-stage completely stirred tank reactors at 35 ± 1 °C. System stability and the effect of organic loading rate (OLR), sludge retention time (SRT) and total solid (TS) content on the performance of high-solid system was examined. Experimental results showed that, with the concentration of free ammonia nitrogen (FAN) lower than 600 mg l 1 , high-solid anaerobic digestion of sewage sludge could maintain satisfactory stability. Slight, moderate and significant inhibition was found with FAN concentration ranging from 250 to 400, 400 to 600 and 600 to 800 mg l 1 , respectively. The VFA/TA criteria could not foresee system instability in sig- nificant ammonia inhibition system by its traditional ratio grades. High-solid system could support higher OLR (4–6 times as high) and obtain similar methane yield and VS reduction as conventional low-solid system at the same SRT, thus reach much higher volumetric methane production rate. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Volume and mass of sludge generated in waste water treatment plants (WWTPs) are expected to increase continuously in the next decade, due to increasing population connected to sewage net- works, building of new WWTPs and upgrading of existing plants to fulfill the more stringent local effluent regulations. In China, the annual production of sewage sludge (80% moisture content) has reached almost 3000 tons, and 80% of it has not obtained nec- essary stabilization. Thus, it is essential to develop proper treat- ment processes to reduce the amount of sludge. One of the most widely used processes is anaerobic digestion or methanisation. It can realize sludge stabilization by converting a part of its organic matter into biogas which is a renewable energy source. This tech- nology has been successfully implemented in the treatment of agricultural wastes, food wastes, and sewage sludge (Chen et al., 2008). However, the use of traditional anaerobic digester for low-solid sewage sludge is not always feasible in small-scale WWTPs or WWTPs in some undeveloped countries. For example, up to 2010, among the 2500 WWTPs in China, only 50 ones were designed with anaerobic digestion systems, and only 20% of them were well oper- ated. Low-solid anaerobic digestion was not well applied in China mainly due to poor management, unprofessional operation, economical limitation and inadequate planning. However, more than 80% of the sewage sludge in China has already been dewatered before further disposal or treatment, which makes it favorable to be centralized processed. Hence, high-solid anaerobic digestion could be one viable option to solve the sludge disposal problems in undeveloped countries like China or in small-scale WWTPs. High-solid anaerobic digestion is usually characterized by a high TS content of the feedstocks, typically greater than 15% (w/w) (Rapport et al., 2008) and has been claimed to be advantageous over traditional low-solid anaerobic digestion for several reasons, such as smaller reactor volume, lower energy requirements for heating, less material handling, and so on (Guendouz et al., 2008). So far, a wide range of organic solids found in municipal, indus- trial, and agricultural wastes have been investigated as feedstocks in high-solid anaerobic digestion, including food wastes (Cho et al., 1995; Lu et al., 2007), agricultural wastes (He et al., 2008; Lis- sens et al., 2004; Mosier et al., 2005; Pang et al., 2008), and organic fraction of municipal solid wastes (OFMSW) (Bolzonella et al., 2006; Forster-Carneiro et al., 2007; Martin et al., 2003; Sans et al., 1995). However, so far, no reports can be found focusing on high-solid anaerobic digestion of sewage sludge with feeding TS of 20%. Fuji- shima et al. (1999) has suggested a system in which the dewatered sludge discharged from small-scale plants is collected and sent to a plant with an anaerobic digester. They investigated the effect of moisture content on anaerobic digestion of dewatered sludge, but the TS contents of the fed sludge used in their study were below 11% (w/w), and a long-term effect was not investigated. Nges and Liu, 2010 investigated the effect of solid retention time (SRT) on anaerobic digestion of dewatered sludge in mesophilic and thermo- philic conditions in long-term experiments, but the TS content of fed sludge was below 12%. Up to now, there are few studies related 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.10.090 Corresponding author. Tel.: +86 21 65981794; fax: +86 21 65983602. E-mail address: [email protected] (B. Dong). Bioresource Technology 104 (2012) 150–156 Contents lists available at SciVerse ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

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Page 1: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

Bioresource Technology 104 (2012) 150–156

Contents lists available at SciVerse ScienceDirect

Bioresource Technology

journal homepage: www.elsevier .com/locate /bior tech

High-solid anaerobic digestion of sewage sludge under mesophilic conditions:Feasibility study

Nina Duan, Bin Dong ⇑, Bing Wu, Xiaohu DaiNational Engineering Research Center for Urban Pollution Control, School of Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, PR China

a r t i c l e i n f o

Article history:Received 10 September 2011Received in revised form 22 October 2011Accepted 25 October 2011Available online 3 November 2011

Keywords:Anaerobic digestionSewage sludgeHigh-solidMesophilic

0960-8524/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.biortech.2011.10.090

⇑ Corresponding author. Tel.: +86 21 65981794; faxE-mail address: [email protected] (B. Dong).

a b s t r a c t

Feasibility of high-solid anaerobic digestion of sewage sludge was investigated in single-stage completelystirred tank reactors at 35 ± 1 �C. System stability and the effect of organic loading rate (OLR), sludgeretention time (SRT) and total solid (TS) content on the performance of high-solid system was examined.Experimental results showed that, with the concentration of free ammonia nitrogen (FAN) lower than600 mg l�1, high-solid anaerobic digestion of sewage sludge could maintain satisfactory stability. Slight,moderate and significant inhibition was found with FAN concentration ranging from 250 to 400, 400 to600 and 600 to 800 mg l�1, respectively. The VFA/TA criteria could not foresee system instability in sig-nificant ammonia inhibition system by its traditional ratio grades. High-solid system could supporthigher OLR (4–6 times as high) and obtain similar methane yield and VS reduction as conventionallow-solid system at the same SRT, thus reach much higher volumetric methane production rate.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Volume and mass of sludge generated in waste water treatmentplants (WWTPs) are expected to increase continuously in the nextdecade, due to increasing population connected to sewage net-works, building of new WWTPs and upgrading of existing plantsto fulfill the more stringent local effluent regulations. In China,the annual production of sewage sludge (80% moisture content)has reached almost 3000 tons, and 80% of it has not obtained nec-essary stabilization. Thus, it is essential to develop proper treat-ment processes to reduce the amount of sludge. One of the mostwidely used processes is anaerobic digestion or methanisation. Itcan realize sludge stabilization by converting a part of its organicmatter into biogas which is a renewable energy source. This tech-nology has been successfully implemented in the treatment ofagricultural wastes, food wastes, and sewage sludge (Chen et al.,2008).

However, the use of traditional anaerobic digester for low-solidsewage sludge is not always feasible in small-scale WWTPs orWWTPs in some undeveloped countries. For example, up to 2010,among the 2500 WWTPs in China, only 50 ones were designed withanaerobic digestion systems, and only 20% of them were well oper-ated. Low-solid anaerobic digestion was not well applied in Chinamainly due to poor management, unprofessional operation,economical limitation and inadequate planning. However, morethan 80% of the sewage sludge in China has already been dewatered

ll rights reserved.

: +86 21 65983602.

before further disposal or treatment, which makes it favorable to becentralized processed. Hence, high-solid anaerobic digestion couldbe one viable option to solve the sludge disposal problems inundeveloped countries like China or in small-scale WWTPs.High-solid anaerobic digestion is usually characterized by a highTS content of the feedstocks, typically greater than 15% (w/w)(Rapport et al., 2008) and has been claimed to be advantageous overtraditional low-solid anaerobic digestion for several reasons, suchas smaller reactor volume, lower energy requirements for heating,less material handling, and so on (Guendouz et al., 2008).

So far, a wide range of organic solids found in municipal, indus-trial, and agricultural wastes have been investigated as feedstocksin high-solid anaerobic digestion, including food wastes (Choet al., 1995; Lu et al., 2007), agricultural wastes (He et al., 2008; Lis-sens et al., 2004; Mosier et al., 2005; Pang et al., 2008), and organicfraction of municipal solid wastes (OFMSW) (Bolzonella et al., 2006;Forster-Carneiro et al., 2007; Martin et al., 2003; Sans et al., 1995).However, so far, no reports can be found focusing on high-solidanaerobic digestion of sewage sludge with feeding TS of 20%. Fuji-shima et al. (1999) has suggested a system in which the dewateredsludge discharged from small-scale plants is collected and sent to aplant with an anaerobic digester. They investigated the effect ofmoisture content on anaerobic digestion of dewatered sludge, butthe TS contents of the fed sludge used in their study were below11% (w/w), and a long-term effect was not investigated. Nges andLiu, 2010 investigated the effect of solid retention time (SRT) onanaerobic digestion of dewatered sludge in mesophilic and thermo-philic conditions in long-term experiments, but the TS content offed sludge was below 12%. Up to now, there are few studies related

Page 2: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

N. Duan et al. / Bioresource Technology 104 (2012) 150–156 151

to high-solid anaerobic digestion of sewage sludge with TS above12% in a long-term operation. In this study, high-solid mesophilicdigestion of sewage sludge (feeding TS 20%) was investigated usingsingle-stage completely mixed reactors (semi-continuously oper-ated), the performance of which were compared with reactors fedby TS of 10% and 15%. The effect of OLR, SRT and TS content wasexamined, with special attention paid to ammonia inhibition andsystem stability in high-solid state. The aim was to investigatethe stability of high-solid anaerobic digestion of sewage sludgeand to evaluate its performance.

2. Methods

2.1. Substrates and inoculums

Dewatered sewage sludge from Anting WWTP (Shanghai, Chi-na) was used as substrate for the present study. The sludge was ob-tained by collecting primary and excess sludge and dewatered withthe aid of a high-molecular flocculant based on polyacrylamide.The total solids (TS) of the dewatered sludge ranged from 19% to23% (w/w) and volatile solids (VS) accounted for 50–60% of TS.The mesophilic seed sludge collected from an anaerobic digesterat Bailonggang WWTP (Shanghai, China) had TS of 2.3% (w/w)and VS was 56.5% of TS. Characteristics of dewatered sludge andinoculums are listed in Table 1. The collected sludge was storedat 4 �C and heated to 35 �C before everyday feeding.

2.2. Reactors and operation

Three identical reactors (numbered R1, R2 and R3), with liquidworking volume of 6.0 l, were equipped with helix-type stirrers,which were set at a rate of 60 rpm (rotations per minute) with10 min stirring and 10 min break continuously. Volumes of pro-duced biogas were measured by wet gas meters every day.

On the first day of the experiments, 6.0 l seed sludge was addedto each reactor, which was operated semi-continuously (once-a-day draw-off and feeding) at 35 ± 1 �C. During the start-up period,when the TS concentration of the substrate in the reactor did notreach its designed level, the OLR was increased stepwise withdewatered sludge as feedstock. Once the TS of the substrate in eachreactor approached its designed level, the feeding sludge was di-luted to its designed TS level (10%, 15% and 20%, respectively) withde-ionized water before feeding.

2.3. Analytical methods

Biogas and substrate samples of the reactors were taken twice aweek and analyzed for methane content, pH, TS, VS, volatile fattyacid (VFA), total alkalinity (TA), total ammonia–nitrogen (TAN)and free ammonia–nitrogen (FAN). Methane content of the biogaswas measured by a gas chromatograph (GC) (Agilent Technologies6890N, CA, USA) with a thermal conductivity detector equippedwith Hayseq Q mesh and Molsieve 5A columns. To analyze VFAs,the sludge samples from the reactors were centrifuged at

Table 1Characteristics of the substrate and inoculums (TS and VS values reported are averages of

Parameters DSa 1 (days 1–55) DS 2 (days 56–115) DS 3 (days 11

pH 7.4 7.4 7.5TS (%, w/w) 20.45 22.56 23.08VS/TS (%) 60.09 60.29 52.48TAN (mg l�1) 783 694 801FAN (mg l�1) 26 23 33

a Dewatered sludge.

10,000 rpm for 10 min. Then the supernatant was passed througha microfiber filter (0.45 lm) and the filtrate was acidified by formicacid to adjust the pH to approximately 2.0 before VFA was ana-lyzed by a GC (Agilent Technologies 6890N, CA, USA) with flameionization detector. TS, VS, TA and TAN were determined accordingto standard methods (APHA, 1995). FAN concentration was calcu-lated in the same way as described by Østergaard (1985). The deg-radation or removal level based on VS (i.e. VS reduction) wascalculated by the same equation as reported by Koch et al.(2009), assuming that the mass of undegradable material (inor-ganic fraction) is constant:

VSreduction ¼ 1� VSdigestate � 1� VSfeedð ÞVSfeed � 1� VSdigestate

� � ½%�; ð1Þ

where VSdigestate is the loss on ignition of digestate (% of TS) andVSfeed is the loss on ignition of feeding sludge (% of TS).

3. Results and discussion

3.1. Start-up performances

Figs. 1–3 show the performance data of R1, R2 and R3, whichwere fed with dewatered sludge until each reached its designedTS level (on day 30, 68 and 85, respectively). Then the feedingsludge was diluted to its designed TS level (10%, 15% and 20%,respectively) with de-ionized water before feeding. In the first17 days, each reactor was operated at an OLR of 4.1 kg VS m�3 d�3,which was much higher than that in traditional low-solid digestionsystem for sewage sludge (Metcalf and Eddy, 2003; Qasim, 1999;Turovskiy and Mathai, 2006). Significant VFA accumulation andsharp increases of VFA/TA ratio were observed, accompanied bydecreasing biogas production (days 5–20) and VS reduction (days12–20), which indicated low stability at high starting OLR, proba-bly due to the small amount of biomass and its poor acclimationto the substrate at the beginning.

As OLR reducing to 2.0 kg VS m�3 d�3, methane yield of eachreactor showed increasing trend with great fluctuation, probablydue to the conversion of accumulative VFAs to biogas. From day20 to the end of the start-up period (day 30, 68 and 85, respec-tively), the TA, VS reduction and TAN concentration in each reactorshowed increasing trend with TS increasing (the drop of TAN con-centration on day 20 may be caused by the initial drop of VS reduc-tion), indicating better buffering capacity and successful recoveryof the systems.

It should be noticed that, although the system was inoculatedby digestate from traditional low-solid digester treating sewagesludge and experienced intense unstable state (VFA/TA 0.8-1.1)by overloading, it could still successfully recover in continuousoperation at OLR 2.0 kg VS m�3 d�3, which was still higher thanthe regular OLR applied in low-solid digesters treating sewagesludge. The results indicated that, as long as pH was not dramati-cally influenced and was kept in favorable range for methanogens,previous high VFA/TA state was quite recoverable in continuous

three measurements).

6–141) DS 4 (days 142–160) DS 5 (days 161–200) Inoculums

7.4 7.6 7.819.72 22.35 2.3152.47 51.17 56.51

752 739 32125 38 26

Page 3: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

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Fig. 1. Performance data of R1 (feeding TS of 10%).

152 N. Duan et al. / Bioresource Technology 104 (2012) 150–156

operation, because VFA accumulation would help providing suffi-cient substrate for multiplication of methanogens to same extent.

3.2. Ammonia inhibition and system stability

It should be noticed that, at the end of the start-up period of R2,the VFA concentration was still relatively higher than normal level.At the end of the start-up period of R3, the VFA concentrationraised rapidly to 4500 mg l�1 at day 85, with a dramatic decreaseof methane yield. That may probably due to the decreasing meth-anogenic activity caused by ammonia inhibition (De Baere et al.,1984; Hansen et al., 1998; Lay et al., 1997). High VFA levels and al-most steady VS reduction in this period indicated that the acido-genic activity was not influenced significantly, which was inaccordance with the report of Koster and Lettinga (1988).

To relieve the dual inhibition caused by ammonia and high VFA,the OLR of Reactor 3 was reduced to 2.0 kg VS m�3 d�3 after day 85.Since free ammonia has been suggested to be the actual toxicagent, the concentrations of both TAN and FAN are shown in Figs.1–3.

It was obvious that methanogens were not evidently influencedwith TAN 1000–2500 mg l�1 and FAN lower than 250 mg l�1

(Fig. 1), since no VFA accumulation and deterioration in biogasproduction (the sharp decrease on day 116 was due to the VS/TSchange in feeding sludge) was shown. Evident influence was

exhibited at FAN 500–600 mg l�1 (Figs. 2 and 3), which led to slightaccumulation of VFAs. Significant inhibition occurred with FAN600–800 mg l�1 (Fig. 3), leading to intense VFA accumulation anddramatic drop of biogas production.

The results suggested that, in high-solid anaerobic digestion ofsewage sludge, free ammonia concentration was an important fac-tor influencing the stability of the system. Taken into considerationof VFA accumulation and biogas production as the main factors toevaluate the real stability degree (VFA accumulation without dra-matic drop of biogas production was considered steady state, anddramatic drop of biogas production was considered a sign of greatinstability), the inhibitory effect of free ammonia can be classifiedto several degrees as listed in Table 2.

As shown in Table 2, in a high-solid anaerobic system, whereboth free ammonia and VFA were important factors affecting meth-anogenic activity, the parameter of VFA/TA ratio was not feasible toevaluate the stability of the system. In the situation of ‘slight inhi-bition’, system stability was affected mainly by VFA concentration,and successful recovery was proved at high OLR (2.0 kg VS m�3 d�3)with VFA accumulation up to 10 g l�1 (see start-up performance)when the VFA/TA ratio was near 1.0. Once full recovery was ob-tained from overloading, the VFA/TA ratio would decrease to0.03–0.12 in the ‘slight inhibition’ situation. In the situation of‘moderate inhibition’, slight VFA accumulation existed as a resultof decreased methanogenic activity caused by ammonia inhibition,

Page 4: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

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0.00.10.20.30.40.50.60.70.80.91.01.11.2

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/ TA

Fig. 2. Performance data of R2 (feeding TS of 15%).

N. Duan et al. / Bioresource Technology 104 (2012) 150–156 153

but the VFA/TA ratio was still below 0.2 and the system was in a‘inhibited steady’ state. As the inhibitory effect reached ‘significant’degree, both FAN and VFA concentration rose to higher levels, lead-ing to great instability of the system. However, the VFA/TA valuewas still below 0.3, which was used to indicate good stability in tra-ditional low-solid digestion systems (Switzenbaum et al., 1990).The result suggested that, in significant ammonia inhibited system,VFA/TA criteria can no longer foresee system instabilities, at leastby its traditional ratio grades.

The reasons are: in traditional low-solid digestion system, sys-tem instability comes mainly from the decreasing methanogenicactivity caused by accumulated VFAs and the drop of pH it induced.Since the VFA/TA ratio, where VFA concentration represents the fac-tor influencing system stability and TA concentration representsthe buffering capacity, can properly reflect the result of the interac-tion between them, it is then able to evaluate system stability intraditional low-solid digestion system. While in ammonia inhibitedsystem, system instability comes from two aspects, one of which isammonia inhibition, the other is the same as that in low-soliddigestion system. When FAN concentration increased, it contrib-utes more TA (see Eq. (2)) while increases VFA concentrations(within specific range of FAN concentrations, VFA accumulates tocertain level, then causes a decrease of pH, which reduces FAN con-centration, and the system then can reach a new steady state). VFA/TA ratio could not properly reflect the stability caused by ammonia,

thus could no longer accurately foresee system stability in high-so-lid system, where ammonia inhibition showed more significantinfluence than the other aspects.

NH3 þH2Oþ CO2 ! NHþ4 þHCO�3 ð2Þ

3.3. Effect of OLR and SRT

After start-up, all of the three reactors ran at their designedfeeding TS levels; hence their performance parameters at certainOLR or SRT were clearly demonstrated. Table 3 indicates the aver-age performance data of each reactor at certain OLR or SRT afterreaching steady state. Since some of the operational conditionsdid not last more than one SRT, the performance data in the laterperiod of each operational condition was chosen for calculationof an average value. It should be mentioned that, in the first115 days, the VS/TS (%) of the feeding sludge was about 60%, butfrom day 116 the VS/TS of the feeding sludge used was about52%, due to the change in waste water quality of the WWTP. Thesudden change of VS/TS led to measurement errors in VS reductionsince the VS of the digestate was higher than its real value in thefollowing one or two SRTs of each reactor.

The performance of high-solid system with feeding TS 20% canbe characterized by comparisons as discussed below:

Page 5: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

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0.00.10.20.30.40.50.60.70.80.91.01.11.2

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ded ),

VFA

/ TA

Fig. 3. Performance data of R3 (feeding TS of 20%).

Table 2Inhibition description with different ranges of FAN concentration.

Inhibition description FAN (mg l�1) TAN (mg l�1) VFA (mg l�1) VFA/TA Process stability

Slight or no <250 <2000 <400 0.02–0.04 SteadySlight 250–400 2000–4000 400–800 0.03–0.12 SteadyModerate 400–600 3000–4000 1000–3000 0.05–0.20 Inhibited steadySignificant 600–800 3000–4000 3000–4500 0.19–0.26 Fragile

154 N. Duan et al. / Bioresource Technology 104 (2012) 150–156

i. Performance comparison with other reports: The performancedata presented in Table 3 exhibited that with an increase ofOLR or decrease of SRT, the pH value, CH4 content, VS reduc-tion and methane yield showed decreasing trend, while thevolumetric methane production rate increased. The perfor-mance values of the three reactors on VS reduction andmethane yield were relatively lower than that in traditionallow-solid digestion of sewage sludge (Metcalf and Eddy,2003; Qasim, 1999; Turovskiy and Mathai, 2006) (methaneyield 0:3� 0:5 l CH4 g�1 VS�1

added d�1, VS reduction 50–65% atOLR 0.64–1.6 kg VS m�3 d�3, HRT 20–60 d) and thosereported by some other researchers (De la Rubia et al.,2006; Fujishima et al., 1999; Nges and Liu, 2010), whichmay attributed to the differences in substrate degradabilityinstead of TS content. As is shown in Table 3, VS reduction

and methane yield of R3 were better with Sludge 5 (VS/TS51%) than Sludge 4 (VS/TS 52%), even at higher OLR andshorter SRT. That may due to the difference in hydrolysisvelocity, since the hydrolysis constants estimated (by themethod reported by Koch et al., 2009) for Sludge 5 and Sludge4 were 0.45 and 0.33 d�1, respectively.

ii. Performance comparison at the same OLR: The VS reduction,methane yield and methane production rate of each reactorat OLR of 3.0–3.5 kg VS m�3 d�3 or 5.0 kg VS m�3 d�3 couldbe compared. Both of them showed increasing trends withthe feeding TS, but the methane yield and methane produc-tion rate at feeding TS of 20% slightly declined or just close tothat at 15% feeding TS, which was probably due to the influ-ence of ammonia. It should be mentioned that, at high OLRof 8.5 kg VS m�3 d�3, methane yields in R2 and R3 were both

Page 6: High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study

Table 3Performance data of the three reactors after start-up.

Reactor Feeding VS/TS (%) OLR (kg VS m�3 d�3) SRT (d) pH VFA (g l�1) CH4 (vol.%) VSra (%) Y ðl CH4 g�1 VS�1added d�1Þ MPRb (l CH4 l�1 d�1)

R1 feeding TS 10% 60 2.0 30.0 8.0 0.291 72.2 41.7 0.27 0.552.4 25.0 7.9 0.175 71.0 37.6 0.24 0.563.0 20.0 7.8 0.384 67.9 33.3 0.23 0.683.5 17.1 7.8 0.194 67.7 34.7 0.24 0.87

52 4.0 13.1 7.6 0.151 67.5 23.0 0.18 0.715.0 10.5 7.6 0.229 64.7 23.6 0.18 0.906.0 8.7 7.3 0.300 63.6 22.4 0.16 0.99

51 8.5 6.0 7.4 0.573 64.5 20.5 0.15 1.2512.8 4.0 7.3 1.475 61.2 18.6 0.12 1.49

R2 feeding TS 15% 60 3.0 30.0 8.2 1.267 68.0 39.2 0.25 0.753.5 25.7 8.1 0.675 66.8 37.6 0.25 0.87

52 4.0 19.6 8.0 0.218 65.8 21.7 0.19 0.784.5 17.5 7.8 0.102 65.9 24.5 0.22 1.005.0 15.7 7.6 0.159 66.3 28.2 0.20 1.01

51 6.4 12.0 7.8 0.212 66.6 30.0 0.19 1.228.5 9.0 7.7 0.250 66.2 28.1 0.19 1.61

R3 feeding TS 20% 60 2.0 59.1 8.3 2.246 66.5 40.4 0.24 0.493.0 40.0 8.3 3.579 65.9 38.7 0.22 0.67

52 3.0 24.8 8.2 2.015 65.8 19.8 0.21 0.634.0 26.1 8.1 1.005 65.6 23.6 0.20 0.815.0 20.7 7.9 0.813 64.1 28.6 0.18 0.92

51 6.8 15.0 8.0 0.892 65.6 29.7 0.18 1.248.5 12.0 7.9 0.750 64.9 29.0 0.19 1.63

a VS reduction.b Volumetric methane production rate.

N. Duan et al. / Bioresource Technology 104 (2012) 150–156 155

0:19 l CH4 g�1 VS�1added d�1, much higher than that in R1. It

suggested that at the same OLR, high-solid anaerobic diges-tion could support longer SRT, thus higher VS reduction andmethane yield could be obtained. In addition, the compari-son proved that with lower FAN concentration, high-solidsystem could obtain better performance.

iii. Performance comparison at the same (or similar) SRT: The datain R1 and R2 with a SRT of 30 d showed that at feeding TS of15%, values of VS reduction and methane yield were slightlylower than that at feeding TS of 10% at the same SRT, but thevolumetric methane production rate was much higher,which was due to the digestion at higher feeding TS receivedhigher OLR. Comparisons between R2 and R3 at about 20-day SRT (19.6 d for R2 and 20.7 d for R3) and 12-day SRTshowed similar results. The results suggested that, anaerobicdigestion at higher TS content could support higher OLR andobtain much higher volumetric methane production ratewithout (or with only slight) decline in VS reduction andmethane yield.

It can be calculated that, if operated at the same SRT, high-solidanaerobic digestion system could receive an OLR 4–6 times as highas that of traditional low-solid anaerobic digestion, with anassumption that the TS of high-solid and low-solid feeding sludgewas 20% and 3–5% respectively.

The results demonstrated that, with FAN concentration lowerthan 600 mg l�1, high-solid anaerobic digestion of sewage sludgeunder mesophilic conditions was feasible and highly practical inlarge-scale application. Since ammonia was produced by the bio-logical degradation of the nitrogenous matter, mostly in the formof proteins, the protein content, VS reduction, as well as the feed-ing TS of the sludge together decided the total ammonia concentra-tion in anaerobic digesters. That means, to avoid high TANconcentration in the digester, high-solid anaerobic digestion ismore suitable to be applied in feeding sludge with lower proteincontent and operated at high OLRs (short SRTs). In addition, sincetemperature and pH significantly influence the free ammoniaconcentration, high-solid anaerobic digestion is more suitable to

be applied under mesophilic conditions and operated at low pHvalue (again short SRT was favorable).

4. Conclusions

High-solid anaerobic digestion treating dewatered sludge (feed-ing TS 20%) was approved feasible in single-stage completely-mixed semi-continuous reactors under mesophilic conditions.FAN concentration higher than 600 mg l�1 was the main factorinfluencing system stability. The VFA/TA criteria could not foreseesystem instability in significant ammonia inhibition system by itstraditional ratio grades. High-solid system could support higherOLR (4–6 times as high) and obtain similar methane yield and VSreduction as conventional low-solid system at the same SRT, thusreach much higher volumetric methane production rate.

Acknowledgements

This work was financially supported by the National Key Tech-nologies R&D Program of China (2010BAC67B04) and the key pro-jects of National Water Pollution Control and Management ofChina (2011ZX07316-004).

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